Reverse proxy to unbounce landing page suddenly stopped working


I know this isn’t exactly a fully supported use-case of unbounce but it is (or at least was) possible, as we did have one working successfully for a number of months prior to now, but it suddenly stopped working sometime on November 13th or 14th. Now our IIS “Rewrite” rule request to our Unbounce landing pages only returns a “400/bad request” error. I know we had confirmed that it was still working after our last updates to our site prior to this date.

We were wondering if there was a change on Unbounce’s side that might have disrupted this. Has anyone else has had anything similar happen or any ideas?
Maybe this is a technical change on Unbounce’s side that prevents using the IIS “Rewrite” rule we’re using, or maybe something like Unbounce blocking our IP?

I’ve confirmed I can still use the same Rewrite rule directed to other pages or domains without much issue, so it kind of feels like Unbounce is denying the request for some reason. We haven’t yet done enough testing to see if we can narrow down the issue.

Edit: As a note, I am aware that Unbounce officially doesn’t support this usage. I’ve seen this community post, etc: Reverse Proxy to Unbounce